Heart failure (HF) is the leading cause of hospitalization in the elderly and occurs in the presence of preserved ejection fraction (EF) (diastolic HF, DHF) in over 50% of cases. DHF is rapidly increasing in prevalence. To date, no therapy is proven to improve symptoms or survival in DHF and thus, survival of patients with DHF has not improved over time. Based on our previous studies and on the tremendous unmet clinical need for an effective therapy for DHF, the broad objective this proposal is to develop a highly novel, cardiac specific and ultimately translatable therapy for DHF based on over-expressing natriuretic peptide receptors (guanylyl cyclase A (GCA)) in the heart. Our broad hypothesis is that augmenting cardiac GCA will ameliorate adverse remodeling and diastolic dysfunction in DHF. Our highly novel approach will utilize the markedly cardiotropic recombinant adeno-associated virus serotype 9 (AAV-9) vector to over-express GCA in the heart in our well established canine model of DHF. Our preliminary studies have established the rationale for over-expression of GCA as a therapy for DHF, established the ability to enhance GCA activity in the heart via AAV-9-GCA gene delivery in the rodent and established the feasibility of gene delivery in the canine. Our proposed Specific Aims outline the strategy to optimize overexpression of GCA in the canine heart via AAV-9 gene delivery, demonstrate that restoration of cardiac GCA in canine DHF abrogates adverse remodeling and diastolic dysfunction and determine the relative and incremental effect of AAV-9-GCA to phosphodiesterase type 5 inhibition. These studies lay the foundation for translation to human DHF in future studies if our hypothesis is proven correct. SPECIFIC AIM 1: Optimize AAV-9-cGCA gene delivery in the normal canine. Hypothesis: Optimized AAV-9- cGCAgene delivery results in increased cardiac GCA protein and activity. SPECIFIC AIM 2: Determine if AAV-9-cGCA increases GCA protein expression and activity and ameliorates adverse LV remodeling and diastolic dysfunction in canine DHF. Hypothesis: AAV-9-cGCA gene delivery results in increased GCA protein and activity, less hypertrophy and fibrosis and improved diastolic function in canine DHF. SPECIFIC AIM 3: Determine the relative and incremental effects of AAV-9-cGCA and PDE-5 inhibition in canine DHF. Hypothesis: Concomitant PDE-5 inhibition and AAV-9-cGCA provides incremental improvement in hypertrophy, fibrosis and diastolic function as compared to AAV-9-cGCA alone or PDE-5 inhibition alone in canine DHF.